Graded index type plastic optical fibers (hereinafter referred to as "GI type POFS") having a radial refractive index distribution in which the refractive index decreases gradually from the center toward the.outer periphery of the optical fiber have a wider frequency bandwidth than step index type optical fibers, and are hence expected to be useful as optical communication media.
In the case of GI type POFs, one having a large numerical aperture (NA) and as small a transmission loss as possible needs to be manufactured for the purpose of improving its bending loss and its coupling loss with the light source. In order to increase NA, GI type POFs must be designed so that the maximum difference in refractive index (.DELTA.n) between the center and the outer periphery of the optical fiber is sufficiently large.
Various methods of making such GI type POFs are known. They include, for example, (1) a method which comprises providing two monomers having different reactivity ratios and giving homopolymers with different refractive indices, placing these monomers in a cylindrical vessel made of a polymer of these monomers so as to cause the polymer to be dissolved and swollen, polymerizing the monomers, and then drawing the resulting product (Japanese Patent Laid-Open No. 130904/'86); (2) a method which comprises preparing a plurality of polymer mixtures from two polymers having different refractive indices at various mixing ratios, spinning these polymer mixtures to form a multilayer fiber, and then heat-treating this fiber to effect interdiffusion between adjacent layers (Japanese Patent Laid-Open No. 265208/'89); and (3) a method which comprises winding films formed of a plurality of binary copolymers having different copolymerization ratios on a core material, and drawing the resulting laminate under heated conditions (Japanese Patent Publication No. 15684/'80).
The GI type POFs made by the above-described methods (1) or (2) have the disadvantage that, since all layers are formed of polymer mixtures, these plastic optical fibers (hereinafter referred to as "POFs") tend to produce a heterogeneous structure due to microscopic phase separation and hence show a large light scattering loss. On the other hand, the GI type POFs made by the method (3) and consisting of styrene-methyl methacrylate copolymers or the like have a large light scattering loss, because the difference in refractive index between the copolymers constituting adjacent layers of the multilayer fiber is too large (e.g., 0.02).
As the methods of making, the above-described method (1) is disadvantageous in that it requires a polymerization step and hence has low productivity. The method (3) is disadvantageous in that foreign matter tends to be introduced when a plurality of films are wound on a core material and in that it is difficult to obtain a concentric circular fiber because thickness discontinuities tend to occur at the joints between film ends.
On the other hand, the method (2) is excellent in that a GI type POF showing few thickness fluctuation can be continuously formed. However, it is difficult to create a gradual refractive index distribution in the POF, because sufficient polymer-to-polymer interdiffusion between adjacent layers cannot be achieved by the post-spinning heat treatment alone. Even if the heat-treating temperature is raised to increase the thickness of the interdiffusion layers and thereby to create a gradual refractive index distribution profile, the fiber drawn during spinning tends to undergo relaxation shrinkage and show variations in fiber diameter. Consequently, light leakage and scattering occur in the parts showing variation in diameter, resulting in an increased transmission loss.